Prenylation is an important posttranslational modification for many proteins whereby attachment of a lipid tail leads to protein localization to cellular membranes. The two enzymes responsible for the majority of prenylation, farnesyltransferase (FTase) and geranylgeranyltransferase type I (GGTase-l), have been proposed to recognize a consensus "CaaX" box motif at the C-terminus of target proteins. However, recent work has shown that this model incompletely describes the specificity of these enzymes. Furthermore, the interactions between the protein substrates and FTase responsible for specificity have not been delineated. I will investigate these issues using mutagenesis and directed evolution to generate a library of FTase variants with altered substrate specificities and kinetic behavior. Analysis of these mutated FTases will provide a more comprehensive understanding of the features that govern binding and catalysis within FTase and may aid in the identification of novel substrates and design of new FTase inhibitors as therapeutic agents. In addition, this work will provide important information and novel tools for studying protein prenylation in vivo, allowing examination of the myriad biological roles of prenylated proteins. [unreadable] [unreadable] [unreadable]